As interconnectivity in integrated circuits increases and the size of functional elements decreases, the dielectric constant of insulator materials embedding the metallic conductor lines in integrated circuits becomes an increasingly important factor influencing the performance of the integrated circuit. Insulator materials having low dielectric constants (i.e., below 3.0) are especially desirable, because they typically allow faster signal propagation, reduce capacitive effects and cross talk between conductor lines, and lower voltages to drive integrated circuits.
Since air has a dielectric constant of about 1.0, a major goal is to reduce the dielectric constant of insulator materials down towards a theoretical limit of 1, and several methods are known in the art for including air into the insulator materials to reduce the dielectric constant of such materials. In some methods, air is introduced into the insulator material by generating nanosized voids in a composition comprising an adequately crosslinked thermostable matrix and a thermolabile (thermally decomposable) portion, which is either separately added to the thermostable matrix material (physical blending approach), or built-in into the matrix material (chemical grafting approach). In general, the matrix material is first crosslinked at a first temperature to obtain a three-dimensional matrix, then the temperature is raised to a second, higher temperature to thermolyze the thermolabile portion, and cured at a third, still higher temperature to anneal and stabilize the desired nanoporous material that has voids corresponding in size and position to the size and position of the thermolabile portion.
In both the physical blending approach and the chemical grafting approach, nanoporous materials with desirable dielectric constants of about 2.5 and below may be achieved. However, while there is typically only poor control over pore size and pore distribution in the physical blending approach, the chemical grating approach frequently poses significant challenges in the synthesis of the polymers and prepolymers and inclusion of various reactive groups (e.g., to enable crosslinking, addition of thermolabile groups, etc.) into the polymers and prepolymers. Moreover, the chemical nature of both the thermolabile portion and thermostable matrix generally limits processing temperatures to relatively narrow windows which must distinguish the crosslinking (cure) temperature, thermolysis temperature and glass transition temperature, thereby significantly limiting the choice of available materials.
In other methods, air or other gas (i.e. voids) is introduced into the insulator material by incorporation of hollow, nanosized spheres in the matrix material, whereby the nanosized spheres acts as a “void carriers”, which may or may not be removed from the matrix material. For example, in U.S. Pat. No. 5,458,709 to Kamezaki et al., the inventors teach the use of hollow glass spheres in an insulator material. However, the distribution of the glass spheres is typically difficult to control, and with increasing concentration of the glass spheres, the dielectric material loses flexibility and other desirable physico-chemical properties. Furthermore, glass spheres are generally larger than 20 nm, and are therefore not suitable for nanoporous materials where pores smaller than 2 nm are desired.
To produce pores with a size substantially smaller than glass spheres, Rostoker et al. describe in U.S. Pat. No. 5,744,399 the use of fullerenes as void carriers. Fullerenes are a form of carbon containing from 32 atoms to about 960 atoms, which are believed to have the structure of a spherical geodesic dome, many of which are believed to occur naturally. The inventors mix a matrix material with fullerenes, and cure the mixture to fabricate a nanoporous dielectric, wherein the fullerenes may be removed from the cured matrix. Although the pores obtained in this manner are generally very uniform in size, homogeneous distribution of the void carriers still remains problematic. Moreover, both Rostoker's and Kamezaki's methods require addition or admixture of the void carriers to a polymeric material, thereby adding essential processing steps and cost in the fabrication of nanoporous materials.
Although various methods are known in the art to introduce nanosized voids into low dielectric constant material, all, or almost all of them have disadvantages. Thus, there is still a need to provide improved compositions and methods to introduce nanosized voids in dielectric material.